1. Field of the Invention
This invention relates to a method and apparatus for performing frequency division exhibiting extremely low phase noise and low broadband noise, which is based on a novel complex regenerative divider (CRD) method employing complex frequency shifters (CFS) realized by double-quadrature multipliers configured in a feedback regenerative configuration.
2. Background of the Related Art
Frequency dividers or prescalers are among the essential building blocks in frequency generation and synthesis and are extensively utilized in these and many other applications. Two types of dividers are predominantly in use—digital and analog. Digital type dividers fall into two groups—static dividers and dynamic dividers. The static dividers are based on flip-flops (primarily D-type latches, but other types as well). The dynamic dividers are based on the regenerative principle similar to the one used with analog regenerative dividers described later, but with less control over the design and with fewer choices. Compared with analog dividers, digital dividers have higher phase noise, higher broadband noise floor and higher power consumption, particularly as the division ratio and frequencies go higher. Static digital dividers cannot operate at the high frequencies at which the regenerative type dividers operate. Also, digital dividers generally have higher electromagnetic interference (EMI) emissions or ingress into nearby circuits due to sharper transition edges. In modem systems demanding ever better performance, analog dividers are becoming the preferred and often the only choice. However, analog dividers have their own deficiencies, some of which are discussed below.
Analog dividers can be either injection locked or regenerative type. Injection locked dividers utilize an oscillator that is super-harmonically locked to a signal, thus dividing the frequency of the signal by the harmonic number. Injection locking can be used not only for frequency division, but also for frequency multiplication by sub-harmonically locking the oscillator to a signal. While these dividers can have very low noise (particularly if based on LC oscillators, as opposed to digital ring oscillators), they are fairly narrow band and are not suitable for wide frequency range applications.
The other type of analog divider is the regenerative type. The regenerative concept was originally introduced by T. W. Horton in 1922. In the last several years the regenerative concept has been receiving renewed attention, primarily due to its superior noise performance. This type of divider operates on a feedback principle, where a closed loop positive feedback system oscillates synchronously with the applied signal. The system uses a frequency translation device, such as a mixer, inside the loop. Provided enough loop gain and a proper loop phase, the system oscillates at a frequency fractionally related to the input frequency.
A classic prior art regenerative divide-by-2 circuit is shown in a block diagram of FIG. 1. As shown, the device employs a mixer 10 and a filter 12 in the feedback loop. The output of the mixer has two equal spectral components—the upper sideband (USB) at the frequency fin+½ fin=3/2fin and the lower sideband (LSB) at fin−½ fin=½ fin. The purpose of filter 12 is to reject the upper sideband and pass only the lower sideband frequency output by the mixer 10. In the device of FIG. 1, filter 12 is a low pass filter. A band pass filter can also be used since the output frequencies falling in the lower ⅓ portion of the frequency band of the filter cannot be used as explained below. In either case, the upper cut-off frequency, fc, of the filter 12 must be below the USB frequency in order to suppress it.
The circuit of FIG. 1 operates in the following manner. The USB is removed from the loop by filter 12 and the only signal surviving and circulating in the loop is the lower sideband LSB at half the input frequency ½ fin. If the phase shift around the loop at this frequency is 0° or 360° (or integer multiples of it) and the closed loop gain is unity, the circulation will be sustained and the loop will in effect oscillate and reach the equilibrium at half the input frequency, i.e. when fout=½ fin, effectively accomplishing the function of divide by 2. The earlier mentioned digital dynamic dividers work on this same principle, with the role of the filter 12 accomplished by the inherent roll-off frequency response of the active devices.
One significant limitation of the circuit of FIG. 1 as well as of most other prior art regenerative solutions is its limited instantaneous bandwidth or frequency range of operation. The BW is limited not only on the high frequency side as any other circuit, but this circuit is also limited on the low frequency side. The lower frequency limit occurs when the USB frequency at the output of the mixer falls within the pass band of the filter 12 and is no longer being suppressed. This creates a DSB condition of two equal level sidebands and ambiguity in the loop acquisition and capturing process, thereby preventing the loop to lock reliably or even at all to any of the two sidebands. This occurs for the output frequencies below one third of the filter cut-off, i.e. for fout≦⅓ fc. The operation of the prior art circuit of FIG. 1 is thus limited to relatively narrow frequency range of less than 3 to 1, or practically not more than one octave. Another limitation of this circuit is the reduced signal to noise ratio (SNR) of the output divided frequency due to a double sideband conversion (DSB) used in the mixer 10. When compared with a single sideband conversion (SSB), the DSB conversion will exhibit a 3 dB lower SNR. That happens because only one of the two converted sidebands is used while the other one is wasted, i.e. half of the converted power is lost resulting in a 3 dB SNR reduction. This SNR loss adds to other circuit implementation losses and of course can not be recovered by any amount of post-mixer gain.
If instead of the LSB, the USB was selected or allowed to run in the loop (e.g., by a high pass filter at 12), the circuit would accomplish a fractional division by ⅔ or multiplication by 1.5.
Another prior art device is shown in the block diagram of FIG. 2. This device does not suffer the reduced SNR associated with the device of FIG. 1 due to the use of a well known SSB conversion realized by two mixers, 40 and 42, driven in quadrature in both the input port and the output return port. The outputs of the two mixers are summed in the combiner circuit 44 where the unwanted USB is canceled. The generation of quadrature signals is accomplished by the all-pass filters H3(s) 50 and H4(s) 52 employed in the feedback path coming from the output 46, and by the all-pass filters H1(s) 54 and H2(s) 56 located at the input path 14. The SSB conversion in this circuit suppresses the unwanted upper sideband and passes only the lower, desired sideband. The lower frequency is not limited by the USB rejection considerations, but rather by the available BW of the quadrature generation circuitry 50 through 56. The filter 18 is not necessary, but may improve the USB rejection at higher frequencies if needed. One downside of the circuit of FIG. 2 is associated with cascading the multiple dividers for the purposes of having higher division ratios, because the quadrature signals do not propagate through the system and cannot be reused (i.e., the signals are lost in this scheme). Therefore, a repeat of all four quadrature generation circuits 50 through 56 is necessary for every additional stage in the cascade, making the cascading and the often required higher division ratios impractical, large and not very cost effective.
Thus, there remains a need for regenerative frequency divider, which exhibit low phase noise and low broadband noise, and which solves the problems of the prior art regenerative frequency dividers noted above.